This invention relates to a developing agent for an electrophotographic copier and more particularly to such a developing agent capable of removing talc which becomes attached to the surface of the photoreceptor.
By way of introduction, processes generally carried out inside an electrophotographic copier are explained first with reference to FIG. 5 wherein a primary charger 1, a development device 2, a transfer charger 4, a paper removal charger 5, a cleaner 6 and an erasing charger 7 are shown around the periphery of a photoreceptor drum 8. The primary charger 1 is for negatively charging the surface of the photoreceptor drum 8. The development device 2 serves to apply a developing agent on the exposed surface of the photoreceptor drum 8 so as to form a visible image from a latent image which was formed by incident light on the negatively charged surface of the photoreceptor drum 8 by means of an optical system (not shown).
In synchronism with the rotation of the photoreceptor drum 8 in the direction of the arrow A, a copy paper sheet is introduced in the direction of the arrow B. This sheet is brought to the image transfer position C by means of a paper guide 3. The transfer charger 4 discharges electric charges of polarity opposite to that of the developing agent when the copy paper sheet passes between the surface of the photoreceptor drum 8 and itself. This discharge causes the developing agent, which has been electrostatically attached to the surface of the photoreceptor drum 8, to move onto the surface of the copy paper sheet.
The paper removal charger 5 provides an AC corona discharge or the like to the copy paper sheet after the image transfer process, thereby weakening the force with which the copy paper sheet is attached to the surface of the photoreceptor drum 8. The cleaner 6 removes and collects the residual developing agent remaining on the surface of the photoreceptor drum 8 by pressing thereonto a cleaning blade or the like. Thereafter, the erasing charger 7 erases the residual charges on the surface of the photoreceptor drum 8 or makes them uniform before it is charged again.
Image receiving media such as copy paper sheets generally contain talc with magnesium silicate or the like as its principal component so as to make their surface smooth or as a filler which prevents ink or the like from spreading. Such talc is easily separated from the paper, and it has strong affinity with organic substances. For this reason, talc particles falling off from copy paper sheets become attached to the surface of the photoreceptor drum by the discharge of the transfer charger. This happens more prominently if the surface of the photoreceptor drum is made of an organic material. Although the surface of the photoreceptor drum is cleaned by a cleaning blade or the like after each transfer process, talc particles, being smaller than the particles of the developing agent, cannot be removed easily especially if the affinity of the surface of the photoreceptor drum is strong.
In summary, talc particles remain on the surface of the photoreceptor drum even after a cleaning process. In the next cycle of the copier operation, such talc particles remaining on the surface of the photoreceptor drum become charged in polarity opposite to the developing agent such that a potential difference results between where talc is attached and where it is not attached, areas where talc is attached attracting the developing agent. Particles of the developing agent thus adsorbed where talc is attached become transferred onto the surface of copy paper sheet at the time of next transfer, producing black dots and black lines and generally affecting the quality of the toner image adversely. Attempts have therefore been made to redesign the paper transfer route or the angle of contact between the incoming paper and the photoreceptor drum but there have been no basic solutions to this problem by mechanical improvements.